We need to identify the trigger conditions:

Is the condition likely to be caused by changing the values in B8 and B10?
Is it likely to be caused by the values in B8 and B10 changing as a result of
changes to another cell?
Is it likely to happen because the value of H5 changes as a result of changes
to another cell.
Do B8, B10, and H5 contain formulas or directly entered numeric values.

We also need to identify how much of an overhead you spreadsheet can stand.
Is your entire workbook, small, medium, large, enormous, or gigantic? (don't
worry too much what these mean exactly<g>) If, for example, your spreadsheet
has 10,000 rows each with 200 cells each of which contains a seven deep nested
IF formula incorporating multiple lookups, we will not want to fire the macro
on every change in every cell.

yes, you can show the message  but only with macro, not with excel.
ex:
if H5 >B10 then msgbox="You have exceeded maximum  RET allowance!"

Try this

Sub test()

If Range("H5").Value > Range("B8").Value Then
MsgBox "You have exceeded the maximum  OB allowance!"
End If
If Range("H5").Value > Range("B10").Value Then
MsgBox "You have exceeded the maximum RET allowance!"
End If

End Sub